SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26 System description
Page 51 of 217
DCD signal behavior
If AT&C1 is set, as it is by default, the DCD module output line is set by default to the OFF state (high level) at
UART initialization. The module then sets the DCD line according to the carrier detect status: ON if the carrier is
detected, OFF otherwise. For voice calls, DCD is set to the ON state when the call is established. For a data call,
there are the following scenarios (see the u-blox AT Commands Manual [3] for the definition of the interface
data mode, command mode and online command mode):
ï‚· Packet Switched Data call: Before activating the PPP protocol (data mode) a dial-up application must
provide the ATD*99***<context_number># to the module: with this command the module switches from
command mode to data mode and can accept PPP packets. The module sets the DCD line to the ON state,
then answers with a CONNECT to confirm the ATD*99 command. The DCD ON is not related to the context
activation but with the data mode
ï‚· Circuit Switched Data call: To establish a data call, the DTE can send the ATD<number> command to the
module which sets an outgoing data call to a remote modem (or another data module). Data can be
transparent (non reliable) or non transparent (with the reliable RLP protocol). When the remote DCE accepts
the data call, the module DCD line is set to ON and the CONNECT <communication baudrate> string is
returned by the module. At this stage the DTE can send characters through the serial line to the data
module which sends them through the network to the remote DCE attached to a remote DTE
The DCD is set to ON during the execution of the +CMGS, +CMGW, +USOWR, +USODL AT commands
requiring input data from the DTE: the DCD line is set to the ON state as soon as the switch to binary/text
input mode is completed and the prompt is issued; DCD line is set to OFF as soon as the input mode is
interrupted or completed (for more details, see the u-blox AT Commands Manual [3]).
The DCD line is kept in the ON state, even during the online command mode, to indicate that the data
call is still established even if suspended, while if the module enters command mode, the DSR line is set to
the OFF state. For more details, see DSR signal behavior description.
For scenarios when the DCD line setting is requested for different reasons (e.g. SMS texting during online
command mode), the DCD line changes to guarantee the correct behavior for all the scenarios. For
instance, for SMS texting in online command mode, if the data call is released, the DCD line is kept to ON
till the SMS command execution is completed (even if the data call release would request the DCD setting
to OFF).
If AT&C0 is set, the DCD module output line is set by default to the ON state (low level) at UART initialization
and is then always held in the ON state.
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